1. Field of the Invention
The invention relates to an MR apparatus which includes an MR coil system which serves to receive MR signals from an examination zone and comprises two similar conductor loops which include capacitances and are interconnected via conductors which include several capacitances, every two neighboring conductors and the conductor loop sections situated therebetween constituting a respective mesh, and also relates to an MR coil system suitable for this purpose.
2. Description of Related Art
An MR coil system of this kind is known from U.S. Pat. No. 4,680,548. The two conductor loops thereof are generally shaped as two equally large coaxial circles which are interconnected by the conductors. This construction gives the MR coil system the appearance of a bird cage, which is why this coil system is also known as a bird cage coil.
A bird cage coil of this kind has as many different resonance frequencies as it comprises conductors. Each resonance frequency is associated with a different resonant mode, the bird cage coil always operating in the resonant mode in which an as uniform as possible field distribution (in the case of operation as a transmitter coil) or a location-independent constant sensitivity (in the case of operation as a receiving coil) is obtained inside the coil. This location-independency of the sensitivity, notably in a plane perpendicular to the coil axis, is advantageous because it facilitates the reconstruction of an MR image from the MR signals received by the MR coil system. However, there are coil arrays which consist of a plurality of usually flat sub-coils and have a signal-to-noise ratio which is better than that of such a bird cage coil.
Therefore, it is an object of the invention to improve an MR apparatus of the kind set forth in this respect. This object is achieved according to the invention in that the capacitances are proportioned so that only a single resonance frequency occurs, that at least two processing channels are provided for the separate processing of the MR signals occurring in the various meshes and for generating MR data which represents the nuclear magnetization distribution in the examination zone, and that there is provided a reconstruction unit for forming an MR overall image from the MR data from the processing channels.
The invention is based on the recognition of the fact that the MR coil system has only a single resonance frequency when the capacitances in the two conductor loops and the interconnecting conductors are proportioned in a given way, and that at this resonance frequency the individual meshes constituting the MR coil system (a mesh comprises two neighboring conductors and the sections of the two conductor loops which are situated therebetween) are decoupled from one another. Because of the decoupling, the signals received from the individual meshes can be independently processed so as to form MR images like in a coil array. A common MR image can be reconstructed from the individual MR images, said common MR image having a particularly high signal-to-noise ratio, notably at the areas within the coil system which adjoin the meshes. The individual meshes have a sensitivity which is strongly location-dependent, i.e. a very high sensitivity for signals from areas neighboring the meshes and a comparatively low sensitivity for signals from more remote areas. In order to obtain an MR image having such a high signal-to-noise ratio throughout the entire outer area, therefore, it is necessary to combine the MR data or MR images acquired from the individual meshes so as to form an MR overall image. The term "MR overall image", however, is to be broadly interpreted. It is to be understood to mean not only a representation of the spatial distribution but also the spectroscopic distribution.
Accordingly, in an MR coil system with two similar conductor loops which include capacitances and are interconnected via a plurality of conductors which include capacitances an improved signal-to-noise ratio is achieved in that for the reception of a plurality of MR signals with a location-dependent sensitivity within the area enclosed by the coil system the capacitances are proportioned so that only a single resonance frequency occurs.
In a further embodiment of the invention, a respective processing channel is provided for each capacitance of one of the two conductor loops. Because a separate processing channel is thus provided for each mesh, the MR overall image reconstructed from the MR data from the individual processing channels has the best possible signal-to-noise ratio. However, substantial means are required since a processing channel must be provided for each mesh.
In another embodiment of the invention, however, a respective combination circuit is provided for every two neighboring capacitances in a conductor loop, a respective processing channel being connected to each combination circuit. In that case a processing channel is required only for every two meshes, because the signals of each time two neighboring meshes are combined via the combination circuit. However, the best possible signal-to-noise ratio will then be achieved only at a few areas of the MR overall image.
As has already been stated, the individual meshes have a strong location-dependent sensitivity which must be determined for the reconstruction of an optimum MR overall image. It is known that this requires an MR coil which has a substantially location-independent sensitivity throughout the entire examination zone. Therefore, a further embodiment of the invention is provided with switching units for switching the capacitances of the MR coil system between a first and a second mode of operation, the MR coil system having only a single resonance frequency in the first mode of operation and several resonance modes with different resonance frequencies in the second mode of operation, the resonance mode with a locally uniform coil sensitivity lying at a resonance frequency which corresponds to the single resonance frequency in the first mode of operation.
Thus, in the first mode of operation the MR coil system operates as a coil array, locally having a high but overall a strongly location-dependent sensitivity, and in the second mode of operation as a conventional bird cage coil having a substantially location-independent mean sensitivity.